This invention relates to a high pressure air gun for generating powerful impulses, and more particularly to a novel air gun which employs a sleeve-type shuttle.
Air guns for producing powerful impulses have many advantages which result from the fact that the powerful impulse is readily repeatable as desired. Such powerful impulses are useful for many purposes, for example, in seismic exploration the air guns are used to generate seismic impulses transmitted into the medium in which they are positioned, such as in water or in a land seismic source, which contains water and is coupled to the earth.
In seismic surveying in a medium, such as water, seismic wave energy is generated by air guns submerged in the water and is utilized to investigate subsurface geological conditions and formations. For this purpose, one or more of such air guns are submerged in the water, and compressed air, or other gas or gases under pressure, is fed to the submerged guns and temporarily stored therein. At the desired instant, the seismic source air guns are actuated, i.e., fired by means of an electrically operated trigger valve, i.e. a solenoid valve, and the pressurized gas is abruptly released into the surrounding water. In this manner, powerful seismic waves are generated capable of penetrating deeply into subsurface material to be reflected and refracted therein by the various strata and formations. The reflected or refracted waves are sensed and recorded to provide information and data about the geological conditions and formations.
It is also possible to submerge such seismic sources in marsh land, swamp or mud areas which are infused with sufficient water that the seismic surveying apparatus described herein can be used. Accordingly, the term "water" as used herein is intended to include marsh land, swamp or mud which contains sufficient water to enable such apparatus to be used.
For further background on such apparatus, reference may be made to U.S. Pat. Nos. 3,249,177; 3,379,273; 3,653,460; 3,808,822; 3,997,021; 4,038,630; and 4,234,052 assigned to the same assignee as the present application.
For further background in air guns used in land seismic sources, reference may be made to U.S. Pat. Nos. 3,310,128; 3,779,335; 3,800,907; and 4,108,271 assigned to the same assignee as the present application.
For further background on solenoid valves for triggering the firing of airguns and for monitoring the instant of firing reference may be made to U.S. Pat. Nos. 3,588,039; 4,210,222; and 4,240,518 assigned to the same assignee as the present application.
In the prior art, a sleeve-type shuttle air gun, such as shown and described in U.S. Pat. No. 3,638,752 to Wakefield entitled "Seismic Signal Generator", includes two high pressure chambers, a control chamber and a firing (or main) chamber. The firing chamber is provided with exhaust ports which in the unfired position are sealed by a movable sleeve shuttle. The sleeve shuttle has an upper (or control) face which is exposed to pressurized air in the control chamber and has a greater exposed area than the lower (or firing) face of the sleeve shuttle which is exposed to the pressurized air in the firing chamber when the sleeve shuttle is in its closed position. When the pressure of the air in both chambers is equalized, the differential in the forces being exerted on the exposed opposite ends of the sleeve shuttle holds the sleeve shuttle in closed or blocking position over the exhaust ports. A solenoid valve is provided for firing the sleeve type air gun by dropping the pressure in the control chamber by dumping the high pressure air contained therein, thereby upsetting the differential in the forces on the two opposing ends of the sleeve shuttle. Thus, the shuttle is moved for opening the exhaust ports to allow the pressurized air in the firing chamber to escape.
This dumping of pressurized gas in order to trigger the firing of an air gun is undesirable because it may create spurious signals in the water when the air gun is being used for marine seismology and also because of the flow limitations and difficulties associated with dumping or venting techniques for high pressure air.
U.S. Pat. No. 4,051,918 to Rogers, entitled "Seismic Energy Source" illustrates a tandem sleeve shuttle air gun arrangement operating on similar principles; namely, the venting of high pressure air is required for firing the first chamber of this tandem air gun. The reduction in pressure in the first chamber in turn causes the second chamber to fire.
With respect to sleeve shuttle type air guns, U.S. Pat. No. 4,114,723 to Paitson entitled "Pneumatic Seismic Signal Generator With Independent Firing Control Pressure" recognizes that there is an advantage in using a single air supply for both the control and firing chambers but states that there are many disadvantages aside from the mechanical complexity of the gun. Since very high air pressures are involved, this Paitson patent states that the solenoid dump valve must be specially built and the solenoid valve seat must be very small in order to reduce the mechanical force acting against the solenoid valve stem. Also, it states that the pressure equalization lines between control and firing chambers are necessarily quite restricted. It states that in one commercial embodiment in present use, the control-air dump valve is a complex two-stage device in which an electrically operated solenoid valve upsets the forces on a differential-area air-actuated pilot valve. The pilot valve in turn upsets the differential forces across the main exhaust valve to trigger the air gun. It states that use of a pilot valve is necessary to reduce the forces acting against the stem of the solenoid valve. It admits that the additional valving of course contributes to increased gun maintenance.
In view of these difficulties, the Paitson patent discloses a seismic signal generator including the complicating expedient of two different pressure levels of compressed air: a high pressure source of about 3,000 to 5,000 pounds per square inch (p.s.i.) is used for filling the firing chamber and a control pressure source of 200 to 1,500 p.s.i. is used for the control chamber. Air is admitted to the control chamber at such relatively low pressure and holds the sleeve shuttle in the closed position, because of differential in the forces acting on the respective exposed end faces of the sleeve, resulting from a valve seat which covers an outer portion of the area of the firing end face, leaving only an inner portion exposed to the firing chamber. The firing chamber is pressurized to a high pressure by the separate high pressure supply, and there is no communication or equalization between the return and the firing chambers. Besides the complexity of requiring two separate supplies of pressurized air at different pressure levels to be connected to the air gun, this prior art sleeve shuttle air gun also has the disadvantage that it requires that air in the control chamber must be dumped into the ambient in order to upset the differential forces to move the sleeve shuttle away from the exhaust ports to release the pressurized air from the firing chamber. There is the further disadvantage in such prior art apparatus that the control air is dumped to ambient through at least one duct that includes a metering orifice to meter the volume of air released from the control chamber within a known time interval. Such a metering orifice inherently slows the dumping of control air and may be subject to clogging.
In another prior art air gun described in U.S. Pat. No 4,180,139 to Walker, entitled "Fluid Operated Seismic Generator", a hollow cylindrical actuator is mounted inside of the housing and is hydraulically or pneumatically driven within the housing in longitudinal strokes first in one direction and then in the other. This actuator has apertures which move past exhaust ports during each stroke. There are movable sealing pads or movable O-ring seals associated with these apertures which slide past the exhaust ports in the housing during each stroke, and consequently these moving sealing devices are subjected to severe stress and buffeting during each firing, leading to maintenance problems. The size of the exhaust ports themselves is inherently limited in order to minimize the wear and tear on the moving sealing devices each time they slide past the exhaust ports during a firing, with pressurized gas rushing past them. This prior art gun has the complicating aspects that the pressurized actuating air (or other fluid) which drives the cylindrical actuator back and forth must be dumped from a chamber at one end of the air gun while pressurized air is simultaneously being introduced into a chamber at the other end of the air gun, and vice versa, during the next stroke of the actuator. Thus, once again it is seen that a dumping operation is required in order to fire such a prior art air gun. The speed of the actuator assembly is limited by the need to dump air from in front of it, and thus the rapidity of opening the exhaust ports is limited. Furthermore, the actuating air is described as being at a lower pressure, supplied through a pressure regulating valve, than the firing-air pressure, and consequently this prior art air gun suffers the disadvantages and piping or hoseline and valving complexities resulting from the need for two different levels of pressurized air for operation.